a. Field of Invention
The present invention relates to a wiring board including an insulating substrate in which electronic components, such as chip capacitors, chip inductors, or chip resistors, are embedded. More particularly, the present invention relates to a wiring board suitable for use in, for example, a multi-layer wiring board or a package for storing semiconductor elements.
b. Description of Related Art
In recent years, various studies have been performed on multi chip modules (MCMs) in which a number of semiconductor elements are mounted on a build-up wiring board. Typically, electronic components, such as chip capacitors, chip inductors, and chip resistors are soldered onto the surface of a wiring layer formed on the surface of the wiring board.
When electronic components are mounted on the surface of the build-up wiring board, since the components require predetermined regions for mounting, a limitation is imposed on miniaturization of the wiring board. Also, depending on the wiring layout for surface mounting, parasitic inductance of the wiring (which is an undesirable property) becomes large. Parasitic inductance of the wiring also hampers incorporation of such wiring boards into electronic devices of higher frequency.
In order to solve the aforementioned problems, various studies have been performed on methods for embedding electronic components in an insulating substrate. Japanese Patent Application Laid-Open (kokai) No. 11-126978 discloses a method in which electronic components are mounted by soldering, in advance, on a wiring board having a transfer sheet formed from metallic foil, and then the electronic components are transferred. However, this method has problems in terms of accuracy in the positions of the mounted components.
Japanese Patent Application Laid-Open (kokai) No. 2000-124352 discloses a multi-layer wiring board in which an insulating layer is built up on electronic components embedded in a core substrate. When an electronic component is embedded in an insulating substrate such as a core substrate, after a space between the substrate and the electronic component is filled with an embedding resin, and an insulating layer and a wiring layer are built up on the substrate, the electrode of the electronic component must be electrically connected to the wiring formed on the insulating layer, through a metalization technique such as electroless plating.
The embedding resin is preferably colored black in order to prevent random reflection of light, which would otherwise raise problems when a wiring pattern is formed on the built-up insulating layer through exposure and development, or to reduce non-uniformity in the color of the resin during curing of the resin. Therefore, carbon or a similar material must be incorporated as a coloring agent into the resin.
However, when carbon, which is conductive, is excessively incorporated into the embedding resin, the insulating property of the resin deteriorates. Therefore, it is important to design the method for coloring the resin black so that random reflection of light is prevented and non-uniformity in color of the resin is reduced during curing of the resin, with the insulating property between electronic components or between wiring portions formed on the insulating layer being maintained.
A problem arising when a wiring board is used under high-frequency conditions is loss of electrical signals in high-frequency regions. In order to reduce such loss of electrical signals, an embedding resin used for embedding electronic components in the wiring board must have low dielectric constant and exhibit low dielectric loss.